Materials conventionally utilized in implants for orthopaedic and/or dental applications have included commercially pure titanium, Ti--6Al--4V and Co--Cr--Mo alloys. These have generally been selected based on mechanical properties, primarily strength under loading. Unfortunately, the use of conventional metals and metal alloys that meet mechanical requirements for bone replacements can result in metal material failure under long-term physiological loading, necessitating the surgical removal of failed bone implants.
Traditional ceramics have long been appreciated for their cytocompatibility. Conventional ceramic formulations of materials such as hydroxyapatite, bioglasses, bioactive glass ceramics, and calcium phosphate have been shown to enhance formation of new bone mineralized matrix. ("Conventional" refers to ceramics having a grain size greater than 100 nm. In contrast, "nanostructured", "nanophase" and "nanomaterial" refer to ceramics having a grain size of less than 100 nm in at least one direction.) Mechanical properties, specifically, ductility and toughness, of these conventional biosubstitutes, however, are generally not comparable to natural bone. Consequently, use of these materials in orthopaedic/dental applications has been limited. As one example, alumina has been used in the treatment of hand and elbow fractures, edentations, and in arthroplasty. There is therefore a need for biomaterials having ductility and toughness comparable to natural bone.
Implants composed of conventional ceramics have also experienced clinical failure. The cause of failure in the case of ceramic implants has been attributed to a lack of direct bonding with bone, that is, insufficient osseointegration. Osseointegration is necessary in order to stabilize orthopaedic/dental prostheses in situ, to minimize motion-induced damage to surrounding tissues, and to increase overall implant efficacy. Insufficient bonding of juxtaposed bone to an orthopaedic/dental implant can be caused by material surface properties that do not support new bone growth, as with implant materials composed of metal or conventional ceramics. The extent of osseointegration between bone and a newly implanted material is influenced by many factors including a number of host tissue responses. Physical and chemical properties of the biomaterial surface control the type and magnitude of cellular and molecular events at the tissue-implant interface.
Adhesion of bone-forming cells, or osteoblasts, to an implant is initially required for osseointegration. However, enhanced adhesion of osteoblasts to material surfaces does not necessarily result in enhancement of the long-term cell functions which lead to osseointegration of orthopaedic/dental implants and, therefore, a successful implant. For example, Dee, et al. immobilized RGDS (Arginine-Glycine-Aspartic Acid-Serine) peptides on glass [Biomaterials, 17 (2): pages 209-15 (1996)]. They observed enhanced osteoblast adhesion but not enhancement of subsequent functions, finding that mineralization on the peptide-modified glass was similar to that on unmodified glass. Osteoblast functions which occur subsequent to adhesion, and which are required for an effective implant, include proliferation, alkaline phosphatase synthesis, and deposition of extracellular matrix calcium. Enhancement of these long-term osteoblast functions on nanophase ceramics has not been reported. Therefore, there is a need for biomaterials having surface properties that enhance these and other long-term osteoblast functions. There is also a need for biomaterials with surface properties that would aid in the formation of new bone at the tissue/biomaterial interface and therefore, improve orthopaedic/dental implant efficacy.